It is believed that the ringdown waveform is dominated by the QNMs
of the compact object remnant. Thus, the detection of overtones from the ringdown signal allows for precision measurements of the characteristic parameters of compact objects like the mass, charge and angular momentum. Various studies suggest that the ringdown signal is dominated by the mode excitations of photons trapped in unstable circular orbits at the photon sphere, namely the photon sphere (PS) modes [1]}-[2]}. These QNMs are directly related to the existence of the PS, and if the compact object is an asymptotically flat BH no other oscillatory mode is excited. For ECOs, on the other hand, although the PS excitations still exist at the early stage of the ringdown signal, as in the case of BHs, they do not belong to the QNM spectrum [3]}, [4]}.
